Instructions
Plug your "Pokemon" game into the Action Replay.
Plug your Action Replay into the top slot of the DS.
Enter the master code at the cheats screen:
94000130 fffb0000
d5000000 000000XX
b21c4d28 00000000
b0000004 00000000
c0000000 0000000b
d7000000 00024620
dc000000 00000006
d2000000 00000000
The second line of the code is what changes depending on the Pokemon you want. Check the official Codejunkies website for what two digits to enter instead of the two X's at the end of the second line. The type of Pokemon that appears in the wild will depend on those two digits.
Press "Start" to submit the code and choose your save file.
Walk into any area where you encounter wild Pokemon, such as a grassy area or a cave, and you will encounter the species that you modified using the Action Replay.